This invention relates to fuel cell power production systems and, in particular, to a multi-stack high-efficiency fuel cell system and method of operating same.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte matrix, which conducts electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
In building fuel cell systems, individual fuel cells are stacked together to form a fuel cell stack. The number of fuel cells determines the power rating of the fuel cell stack. To provide systems with higher power ratings, a number of fuel cell stacks are utilized and the outputs of the fuel cell stacks are combined to provide the desired power output. In certain fuel cell systems, the fuel cell stack(s) may be organized in one or more fuel cell stack modules, each of which includes one or more fuel cell stacks housed in an enclosure or a containment structure.
A multi-stack fuel cell system may include a fuel cell stack module with multiple fuel cell stacks housed within a common enclosure. In a system of this design developed for high temperature fuel cell stacks and, in particular, for molten carbonate fuel cell stacks, a box-like containment structure is employed as the enclosure and the fuel cell stacks may be arranged along the length of the containment structure. Each fuel cell stack within the fuel cell module has inlet manifolds for receiving fuel and oxidant gases needed to operate the fuel cell stack and outlet manifolds for outputting spent fuel and oxidant gases as anode and cathode exhausts from the fuel cell stack. The containment structure of the fuel cell module includes fuel and oxidant gas inlet ports that communicate through ducts with the respective fuel and oxidant gas inlet manifolds of the fuel cell stacks, and fuel and oxidant gas outlet ports that communicate through ducts with the oxidant and fuel gas outlet manifolds.
In internally reforming fuel cells, a reforming catalyst is placed within the fuel cell stack to allow direct use of hydrocarbon fuels such as pipe line natural gas, liquefied natural gas (LNG), liquefied petroleum gas (LPG), bio-gas, methane containing coal gas, etc. without the need for expensive and complex external reforming equipment. In an internal reformer, water and heat produced by the fuel cell are used by the reforming reaction, and hydrogen produced by the reforming reaction is used in the fuel cell. The heat produced by the fuel cell reaction supplies heat for the endothermic reforming reaction. Thus, internal reforming is used to cool the fuel cell stack.
Two different types of internally reforming fuel cell designs have been developed and used. The first type of an internally reforming fuel cell is a Direct Internally Reforming (DIR) fuel cell module, in which direct internal reforming is accomplished by placing the reforming catalyst within an active anode compartment of the fuel cell. The advantage of direct internal reforming is that the hydrogen produced through such reforming is provided directly to the anode. A second type of internally reforming fuel cell utilizes Indirect Internal Reforming (IIR), which is accomplished by placing the reforming catalyst in an isolated chamber within the fuel cell stack and routing the reformed gas from this chamber into the anode compartment of the fuel cell. The advantage of indirect internal reforming is that the reforming catalyst is protected from poisoning by the fuel cell's electrolyte. Three types of internally reforming stack designs are possible: (1) incorporates only the DIR, (2) incorporates only IIR, and (3) incorporates both DIR and IIR.
An internally reforming molten carbonate fuel cell system, also called Direct Fuel Cell (DFC), incorporating both the DIR and IIR, has evolved as the choice for environmentally friendly power generation and is the leading commercial option for green power. Carbonate power plants have lower emissions of greenhouse gases and particulate matter than conventional combustion-based power plants. Carbonate power plants emit little NOx gas, SOx gas, or particulate matter. Carbonate power plants have been designated “ultra-clean” by the California Air Resources Board (CARB).
Current carbonate fuel cell power plants are available in 300 kW, 1.4 MW, and 2.8 MW sizes. These plants are installed worldwide and have delivered approximately 2.8 gigawatt-hours of clean electricity as of August, 2014. Current carbonate fuel cell power plants demonstrate electrical conversion efficiencies of 45% to 50% in simple cycle configuration. Carbonate fuel cell power plants operate at high temperatures, approximately 600 C, resulting in byproduct heat at sufficiently high temperature to be utilized for waste heat recycling applications, such as power generation.
Current carbonate fuel cell power plants achieve overall thermal conversion efficiency of 90% (low heat value or LHV) when both high grade and low grade heats are utilized. Such utilization of heat occurs in, for example, hospitals and university dormitories, where hot water heating load is high. However, for most applications, and particularly for larger installations, the heat load is lower. For low heat load applications, the combined cycle configuration consisting of the baseline power plant and waste heat utilization in steam-engine generator systems boosts efficiency by a few percentage points.
U.S. Pat. No. 6,365,290 discloses a fuel cell system, known as a DFC Turbine (DFC-T) system, having an alternate cycle where heat from a carbonate fuel cell is used in a gas turbine. The system of U.S. Pat. No. 6,365,290 achieves electrical conversion efficiency approaching 60%. To achieve this efficiency, the system requires that the turbine size be matched with the available fuel cell heat. Therefore, each size plant requires a unique size turbine. Additionally, this system requires a high temperature air-to-air heat exchanger resulting in material and cost disadvantages.
An alternative system utilizes two fuel cell stacks connected in series with respect to fuel flow, which can boost electrical efficiency. The fuel first flows to a first (topping) stack and anode exhaust from the first stack then flows through a second (bottoming) stack having a similar configuration to the first stack. This two stack system allows improved fuel utilization of approximately 80%, providing approximately 7% higher overall system energy conversion efficiency than a baseline simple cycle fuel cell system. The two stack system is described in the U.S. Pat. Nos. 8,062,799 and 8,236,458. See Table 1 below for a summary of various features of U.S. Pat. Nos. 8,062,799 and 8,236,458.
TABLE 1U.S. Pat.U.S. Pat. FeaturesNo. 8,062,799No. 8,236,458Dual stacks: first stack receivesXXoxidant from second stack and secondstack receives fuel from first stack.Controlled bypass of fresh fuel fromXXfirst stack to second stack.First and second stacks are IIR andXDIR, respectively.First stack is IIR and DIR and secondXstack is DIR.Carbond monoxide shifting, waterXXrecovery, and methanation of firstanode exhaust.Oxidizer output (oxidizes secondXstack anode exhaust with fresh andfeeds to the second stack) is partlybypassed to the first stack.Anode booster blower for anode sideXpressure control.
FIG. 1 shows a conventional fuel cell system as described in the '458 patent, which outputs partially-spent fuel exhaust from a topping fuel cell stack A to a bottoming fuel cell stack B. The stack A includes a first cathode side 100 and a first anode side 105. The stack B includes a second cathode side 110 and a second anode side 115. The first anode side 105 is coupled to an anode booster blower 120 which increases the pressure of the hydrogen rich exhaust from the first anode side 105 of the topping stack A and conveys it to the second anode side 115 of the bottoming stack B. The stack B may be supplemented with fresh fuel from a fuel source to increase electrical power generation by the stack B. Since the stack A runs most efficiently on fuel utilization of 65% to 75%, the stack B is supplied with 25% to 35% of the original fuel, thus requiring input of additional fresh fuel to the stack B from the fuel source.
Utilizing the above-described fuel cell system, heat and mass studies indicate that fuel utilization in the stack B needs to be restricted to about 60% to 70% due to thermal balance considerations. These studies also indicate that overall fuel utilization needs to be restricted to approximately 80% for thermal management considerations. Therefore, such a system achieves an overall system efficiency of approximately 55% (LHV) on pipeline natural gas with current fuel cell stacks.